name: tw-blazor-css-strategy description: How to style Blazor + FluentUI components in this repo without Tailwind. CSS isolation for every component, global design tokens, a native-root anchor plus targeted ::deep for FluentUI light-DOM children, ::part() for shadow DOM, and a ban on <style> elements in components because prerender HTML-encodes them. Use when authoring or restyling any .razor component, choosing where CSS lives, styling a FluentUI component, or fixing a first-paint style flash. metadata: version: "1.0.1"

Blazor CSS Strategy (isolation everywhere, anchored ::deep for Fluent children)

We do not use Tailwind. The design system is hand-written plain CSS built on global design tokens. This skill is the standard for where component CSS lives and how to scope it.

Every rule must be in a real stylesheet that <head> links, so the browser applies it to the server-prerendered HTML on first paint. That rules out CSS written inside a component.

Two walls (why the rules look like this)

• Wall A — isolation scope. An isolated *.razor.css stamps its scope attribute

([b-xxxxx]) only on native HTML elements the component itself authors. That includes elements it authors inside a child component's ChildContent. A child component's root (FluentStack's div, FluentLayout, FluentNav's <nav>, the fluent-field / fluent-text-input hosts, fluent-menu-list, another of our components' root) is never stamped, so a plain isolated selector cannot match it.

• Wall B — shadow DOM. FluentUI interactive primitives (fluent-button, fluent-text-input,

fields, …) are web components with open shadow roots in v4 and v5. Their internals are reachable only via ::part() + CSS custom properties. No scoping strategy pierces the shadow boundary.

Wall A was re-verified on .NET 11 RC1 (SDK 11.0.100-rc.1.26425.128, FluentUI 5.0.0) in task 290 with a minimal component rendered through HtmlRenderer: the component's own <div> and a <span> it authored inside FluentStack's ChildContent carried b-…; FluentStack's root div, the fluent-field host, fluent-menu-list, FluentLayout's root and a child component's <section> did not. It is Blazor's isolation design, not a bug, so do not expect a framework release to remove it.

The rules

1. Every component's CSS goes in `Component.razor.css` (CSS isolation). The build bundles

them into Web.Spa.styles.css, which App.razor links in <head>.

1. Never author a `<style>` element in a component — not <style>@(@"…")</style>, not

<style>@($@"…")</style>, not a static <style> block, and not MarkupString to dodge the encoding. Under server prerender Blazor writes a C# expression inside <style> as an HTML-encoded text node (newlines → 
, quotes → "). Browsers do not decode entities inside <style>, so the whole block is invalid and every rule is dropped until the page goes interactive and the renderer rewrites the text. That is the unstyled first-paint flash fixed in task 290. Even a static block is CSS the <head> does not know about. The regression test FirstPaint_Given_PrerenderedShell.Components_Should_AuthorNoStyleElements fails the build's test run on any <style> in source/**/*.razor (Razor comments ignored).

1. Styling a FluentUI light-DOM child (Wall A): native anchor + targeted `::deep`. Give the

component a native element to anchor on — the element that already wraps the child, or a native root <div> added for the purpose (display: contents when it must not add a box) — and write .anchor ::deep .fluent-child. Blazor compiles that to .anchor[b-x] .fluent-child: a plain descendant selector, scoped to this component's markup, in the bundle that loads before first paint. Keep ::deep targeted (a named class or a specific element, > where possible), never ::deep *.

1. Brand tokens are global in web-spa/wwwroot/css/tokens.css as CSS custom properties,

linked in <head> before Web.Spa.styles.css. Consume them with var(--twe-*). Tokens are the single source of truth for color, type scale, radius, elevation, chrome heights and status palette. app.css holds only the reset and genuinely global element rules.

1. Styling inside a FluentUI primitive (Wall B): ::part() + CSS custom properties only.
2. No inline `style=` attributes in our markup. A strict CSP (style-src without

'unsafe-inline') blocks them, and they are CSS outside the stylesheet system. The app sends no CSP header today; keep our own markup ready for one. Per-instance variation is a class or a data-* attribute selected in .razor.css (.twe-avatar[data-size=lg]), not generated CSS and not style="--x: …". (FluentUI itself still writes inline styles, for example FluentLayout's height variables and its container-query <style>. That is framework markup we do not control; it renders as raw markup, not encoded text.)

1. Undefined web components: before Fluent's script defines them, fluent-* elements are

unknown inline elements with no box. Where that shifts layout on first paint, reserve the upgraded box with a :not(:defined) rule anchored the same way (the shell's search field does this). The rule stops matching once the element is defined.

1. Theme on first paint. App.razor renders `<body data-theme-color="#55409c"

data-theme="light">; Fluent's beforeStart initializer builds the brand ramp from it before anything is interactive. Do not apply the theme from OnAfterRenderAsync (a fresh browser paints Fluent's default blue first). Chrome colors that must be right before any script runs (the shell's header/footer frame) use --twe-*` tokens, not Fluent's runtime tokens.

Canonical in-repo example (the app shell)

web-spa/components/TimeWarpPage.razor renders FluentLayout, FluentNav (via NavMenu), a native breadcrumb <nav>, and FluentTextInput. Its root is a native <div class="twe-shell"> with display: contents, and every rule is in TimeWarpPage.razor.css:

<div class="twe-shell">
  <FluentLayout Class=@($"{Id} twe-shell__layout")>
    <FluentLayoutItem Area="LayoutArea.Header">
      <header class="twe-appbar">…</header>   @* authored here → stamped *@
    </FluentLayoutItem>
    …
  </FluentLayout>
</div>

/* TimeWarpPage.razor.css */
.twe-shell { display: contents; }
 
/* Fluent roots (Wall A): anchor on the native root, ::deep to the child. */
.twe-shell ::deep .twe-shell__layout { --layout-header-height: var(--twe-appbar-height) !important; }
.twe-shell ::deep .fluent-layout-item[area=header] { background-color: var(--twe-purple); }
.twe-shell ::deep .twe-nav { background: var(--twe-paper-2); }
 
/* Elements this component authors are stamped: plain isolated selectors. */
.twe-appbar { display: flex; align-items: center; }
.twe-breadcrumb__link { color: var(--twe-blue); }

Other examples: FormField.razor.css (.twe-form-field__control ::deep > fluent-text-input), Profile.razor.css (native .twe-profile root, ::deep fluent-menu > fluent-menu-list), LoginPage.razor.css and Card.razor.css (plain isolation).

Decision quick-reference

History

• Wall A: styling a Fluent root needs a native wrapper; blanket ::deep is painful; inline

style= is rejected for CSP; a global stylesheet is rejected for lack of locality. The first shape was co-located <style> blocks scoped by .{Id} or a fixed root class ("Exception B").

• Task 059-002 (2026-06-22) adopted the isolation-first hybrid with Exception B; commit

a2e107d7f2 made TimeWarpPage the canonical <style>@(@"…")</style> example, and it spread to FormField (task 234), LoginPage and Profile.

• Task 290 (2026-10-09): Exception B's <style>@(…)</style> was the cause of the prerender

first-paint flash (0 shell rules before interactive, 24 after). Exception B is retired. Wall A is still real on .NET 11, so Fluent children are reached with an anchored ::deep in the component's own .razor.css instead: the same locality Exception B wanted, in a stylesheet the browser loads before first paint. Static stylesheets in <head> for Fluent children were the alternative; they bring back a global file per area, so they were not chosen.

Notes

• FluentUI v5 did not remove Wall A or Wall B. The strategy is the same across v4 and v5.
• Proof: tests/container-apps/web/web-spa-playwright-tests/first-paint-playwright-tests.cs

measures the shell with JavaScript off, with blazor.web.js blocked, and settled interactive, and requires the same app bar, logo and search boxes and the brand header in all three. CI uploads its screenshots as the playwright-<run> artifact.